1. The Field of the Invention
The present invention relates to x-ray tube devices. In particular, the present invention relates to x-ray tubes manufactured so as to reduce heat transmission to heat sensitive components, thus enhancing x-ray tube performance and longevity.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an evacuated x-ray tube that contains a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk supportably mounted to a spinning rotor shaft which, in turn, is supported by ball bearings contained in a bearing assembly. The rotating anode, rotor shaft, and bearing assembly are therefore interconnected and comprise a few of the primary components of the rotor assembly.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a stream of electrons to be emitted by thermionic emission. A high voltage potential placed between the cathode and anode causes the electrons to form a stream and accelerate towards a target surface located on the anode. Upon approaching and striking the target surface, some of the resulting kinetic energy is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (xe2x80x9cZ numbersxe2x80x9d) are typically employed. The x-rays are then collimated so that they exit the x-ray tube through a window in the tube, and enter the x-ray subject, such as a medical patient.
As discussed above, some of the kinetic energy resulting from the collision with the target surface results in the production of x-rays. However, much of the kinetic energy is released in the form of heat. Still other electrons simply rebound from the target surface and strike other xe2x80x9cnon-targetxe2x80x9d0 surfaces within the x-ray tube. These are often referred to as xe2x80x9cbackscatterxe2x80x9d electrons. These backscatter electrons retain a significant amount of kinetic energy after rebounding, and when they also impact other non-target surfaces they impart large amounts of heat.
Heat generated from these target and non-target electron interactions can reach extremely high temperatures and must be reliably and continuously removed. If left unchecked, it can ultimately damage the x-ray tube and shorten its operational life. Some x-ray tube components, like ball bearings housed in the bearing assembly, are especially sensitive to heat and are easily damaged. For instance, high temperatures can melt the thin metal lubricant that is typically present on the ball bearings, exposing them to excessive friction. Additionally, repeated exposure to these high temperatures can degrade the bearings, thereby reducing their useful life as well as that of the x-ray tube.
These problems related to high temperatures produced in the x-ray tube have been addressed in a variety of ways. For example, rotating anodes are used to effectively distribute heat. The circular face of a rotating anode that is directly opposed to the cathode is called the anode target surface. The focal track comprising a high-Z material is formed on the target surface. During operation the anode and rotor shaft supporting the anode are spun at high speeds, thereby causing successive portions of the focal track to continuously rotate in and out of the electron beam emitted by the cathode. The heating caused by the impinging electrons is thus spread out over a larger area of the target surface and the underlying anode.
While the use of the rotating anode is effective in reducing the amount of heat present on the anode, high levels of heat are still typically present. Thus, cooling structures are often employed to further absorb and dissipate additional heat from the anode. Once absorbed the heat is typically conveyed to the evacuated tube housing surface, where it is then absorbed by a circulated coolant. One example of such an arrangement disposes concentric grooves on the surface of the anode inverse to the target surface. These anode grooves correspondingly receive concentric cooling fins typically formed on a portion of the evacuated tube. The cooling fins are situated in close proximity to the anode grooves such that during tube operation heat is transferred from the anode to the evacuated tube surface via the groove-fin juncture, then absorbed by the circulating coolant.
A related attempt to effectively dissipate heat in x-ray tubes has involved the utilization of more massive anode structures, enabling a given amount of conducted heat to be spread throughout a larger volume than that available in smaller anodes. Unfortunately, larger anodes require correspondingly more massive rotor assemblies to support the increased mass and rotational inertia of the anode. This in turn creates a larger conductive heat path from the anode, through the rotor shaft, and into the bearings in the rotor assembly, thus causing unwanted bearing heating.
The above cooling practices, while effective for general heat removal, can be insufficient by themselves to prevent heat from passing from the anode, through the rotor shaft, and into the bearings - especially in today""s higher power x-ray tubes. As discussed before, this heat is highly detrimental to the bearings, and to other components within the x-ray tube.
Another method to control tube heat has been to provide x-ray tube components with coatings that exhibit improved thermal characteristics. For instance, coatings have been applied to various anode surfaces to enhance heat transfer from the anode.
The use of such coatings has not been completely successful however. For instance, over time the repeated cycles of heating and cooling may cause emissive coatings to flake or spall away from the coated surface. This debris can then contaminate other components within the x-ray tube, and lead to its premature failure. Moreover, there is often a thermal mismatch between the surface of the coated component and the emissive coating, which tends to weaken the bond between the two materials as they thermally expand during use. Again, this leads to undesired flaking and spalling and the consequent contamination of the x-ray tube.
Additionally, the previous placement of emissive coatings on tube components has not addressed the particular need of preventing heat transfer from the anode to the heat sensitive ball bearings housed in the bearing assembly.
What is needed, therefore, is an x-ray tube that withstands the destructive heat produced within it during use, thus protecting its components. Also desired is a method by which heat produced within the anode can be dissipated such that it is directed away from heat sensitive tube components. Also, any solution should avoid problems created by flaking, spalling, or thermal expansion.
It is therefore an overall object of the present claimed invention to provide an x-ray device and method that utilizes an emissive coating to provide improved thermal operating characteristics in the presence of extreme temperatures and temperature fluctuations.
A related objective is to provide an emissive coating that can be applied to areas within an x-ray tube where there exists a need to dissipate heat before it contacts less heat-tolerant tube components, such as rotor bearings.
Yet another objective is to provide an emissive coating that avoids flaking and spalling, and one that possesses thermal expansion properties that are compatible with other tube components.
It is an additional objective of the present invention to provide an emissive coating that will not produce outgassing or similar break down when subjected to the x-ray tube""s high temperature vacuum environment.
These and other objects and features of the present claimed invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. Briefly summarized, the present invention utilizes a metal oxide formulation to form a high emissive coating that can be applied to the surfaces of certain x-ray tube components and thereby improve thermal operating characteristics. In one presently preferred embodiment a titanium oxide-aluminum oxide mixture is utilized as an emissive coating. Preferably, the emissive coating is then applied to the rotor shaft portion of an x-ray tube rotor assembly, using methods well known in the art, such as plasma techniques. The resulting rotor shaft exhibits several desirable characteristics, including a significant increase in its thermal emissivity. Given its location in close proximity to the tube""s high temperature anode, the rotor shaft""s enhanced emissivity due to the emissive coating allows excessive heat to be more efficiently radiated to adjacent cooling structures. This increased heat removal in turn equates to less heat damage being suffered by various heat sensitive tube components as well as a resultant increase in the tube""s operational life.
Additional features of the preferred emissive coating include both an affinity for adherence to, and thermal compatibility with, the surface to be coated in order to prevent flaking and spalling. Presently preferred embodiments also exhibit good vacuum properties, which prevent outgassing or breakdown of the coating within the evacuated x-ray tube during use.